Transcription factors, in a somewhat simplified definition, are
proteins that regulate transcription by binding to specific sequence
elements in regulatory genome regions such as promoters, enhancers etc.

TFClass is a classification of eukaryotic transcription factors
based on the characteristics of their DNA-binding domains. It comprises
four general levels (superclass, class, family, subfamily) and two
levels of instantiation (genus and molecular species). Two of them
(subfamily and factor species) are optional. More detailed explanations
about the classification scheme and its criteria are given
here..